抄録

In the higher order perturbation method<SUP>(1)</SUP>, it is necessary to calculate a higher mode harmonics, namely, a higher mode flux of a static neutron balance equation; a lambda mode eigenvalue equation. Recently, study on the higher mode flux and its eigenvalue has been attracted for instability analysis of a BWR core<SUP>(2)</SUP>. The higher mode flux used in a reactor analysis was usually solved numerically by the deflation method, in which the higher mode flux and its eigenvalue can be obtained through source iterations by removing already obtained lower order modes from an initially assumed neutron flux distribution<SUP>(3)</SUP>. In this procedure, differential terms in the eigenvalue equation were usually treated by the finite difference method (FDM). However, it is necessary to calculate the higher mode flux with a finer mesh width than the fundamental mode, since distribution of higher mode flux spatially oscillates more widely between positive and negative values than that of a fundamental mode and it is considered that the mesh effect that means the difference between the calculated results with a finite mesh width and an infinite small one becomes larger. <BR>These days, the modern nodal method has been widely applied to a reactor analysis, and has made possible to calculate an eigenvalue and flux distribution precisely even with a coarse node width in a short computation time<SUP>(4)(5)</SUP>. <BR>In the present study, one of the nodal method; the polynomial nodal method in which neutron flux and leakage terms are expanded by polynomial functions, was used for a higher mode calculation based on the diffusion theory to perform accurately a higher mode analysis up to more than the second order.